The low weight, low height-for-age, delayed skeletal maturation, and retarded puberty in children with homozygous sickle cell disease are consistent with chronic malnutrition. Voluntary energy intake in sickle cell patients (SS) appears to be similar to that of control subjects with a normal hemoglobin genotype (AA) despite a higher resting metabolic rate (RMR), which suggests a suboptimal nutritional state. Patients may therefore conserve energy by reducing physical activity; this hypothesis was tested by comparing RMR, total daily energy expenditure (TDEE), and physical activity level (TDEE:RMR) in 16 postpubertal boys with sickle cell disease with those in 16 normal control subjects matched for age, sex, and pubertal stage. The RMR of sickle cell patients measured by indirect calorimetry (mean +/- SD: 7.0 +/- 0.9 MJ/d) significantly exceeded that of the normal control subjects (6.3 +/- 0.5 MJ/d; P = 0.018) but TDEE measured by the heart rate method was greater in the control subjects (13.8 +/- 4.9 MJ/d) than in the sickle cell patients (10.5 +/- 2.2 MJ/d; P = 0.034). Physical activity level was 46% greater in control subjects (2.2 +/- 0.8) than in sickle cell patients (1.5 +/- 0.3; P = 0.006). Adjustment for genotype differences in body weight reduced the genotype difference in physical activity level from 0.70 (95% CI: 0.3, 1.1) to 0.6 (95% CI: -0.06, 1.2). Reducing physical activity is a compensatory mechanism in children with an energy deficiency and a similar adaptive response may occur in sickle cell disease.